1. Field of the Invention
The present invention relates to an optical wavelength converter which converts a semiconductor laser beam into light having half the wavelength of the laser beam.
2. Description of the Prior Art
There are different types of known optical wavelength converters. One of such optical wavelength converters is a waveguide passage type optical wavelength converter which has a channel type waveguide passage formed on the top of a rectangular substrate. Another type is an optical wavelength converter which is constituted in the shape of an optical fiber comprising a core of nonlinear optical crystal and a clad surrounding this core and employs the Cerenkov radiation phase matching. As one of these optical wavelength converters, a second harmonics generator (hereinafter referred to as "SHG") is known, which converts the wavelength of incident primary light into a half.
According to the Cerenkov radiation system of the fiber type SHG, it is possible to generate a second harmonics whose optical phase is almost automatically matched. The SHG is applied to a shortwave light generator or the like.
As shown in FIG. 1, the shortwave light generator comprises a semiconductor laser 11, a coupling lens 12, a fiber type SHG 13 having its core surrounded by a glass clad made of nonlinear optical crystal, and an axicon 14. The coupling lens 12 collects and guides the light emitted from the semiconductor laser 11 onto the end face of the SHG 13. The axicon 14 shapes the wavefront of the second harmonics, irradiated after conversion done by the SHG 13, so as to provide the second harmonics in the form of the parallel flux of light.
It is known that DMNP (3,5-dimethyl-1-(4-nitrophenyl)-pyrazole) is used as nonlinear optical crystal for the core of the fiber type SHG 13 according to the Cerenkov radiation system. According to this SHG, the c axis of the DMNP nonlinear optical crystal is set along the fiber axis (in the direction of the extension). Actually, if DMNP crystal is grown as a core in a hollow passage of the axis of a glass capillary used as a clad in a Bridgman furnace, crystal grows along the c axis. That is, the a and b axes lie within a plane perpendicular to the fiber axis, and the c axis is specified in the direction approximately coincident with the fiber axis. When the wavelength of a semiconductor laser beam, 830 to 870 nm, is to be converted by a fiber type optical wavelength converter having such a core crystal orientation, the refractive index n.sup.2.omega. of the core to polarization of the second harmonics is a little greater than the refractive index n.sup..omega. of the core to polarization of the incident primary light. This hinders the desired improvement of the wavelength converting efficiency.